North Dakota law enforcement and the national guard have responded to the latest Standing Rock demonstrations with an aggressive show of force, according to indigenous activists who fear that police violence will escalate after Donald Trump’s inauguration.

Firsthand accounts from Native Americans fighting the Dakota Access pipeline – a large number of whom remain at Standing Rock – along with live footage from the clashes on Martin Luther King Jr Day, suggest that police in riot gear deployed pepper spray, tear gas and other “less than lethal” weapons against unarmed people, in some cases leading to serious injuries.

Some fear that the harsh police tactics at two demonstrations – which activists insist were peaceful – are a sign that law enforcement may be gearing up to clamp down on the massive protests that are likely to emerge if Trump, as expected, moves to approve the oil pipeline.

“It’s gratuitous. It’s just so excessive,” said Irina Lukban, a 22-year-old who said she was hit in the head by the shield of a national guard soldier and likely suffered a concussion. “It’s incredible to see that amount of force.”

The standoffs with heavily armed officers – broadcast for hours on numerous Facebook live streams – offered a reminder that the fight is far from over and that law enforcement is still closely monitoring and impeding the activities of demonstrators camped near the construction site.

Police, who arrested 16 people in the last two days, have continued to defend their actions, claiming that activists were trespassing and rioting.

On Monday, a day of national protests in honor of Martin Luther King Jr, activists said they engaged in peaceful walks toward the pipeline drill pad and to the Backwater bridge, which has been at the center of numerous clashes. The tribe has argued that law enforcement’s refusal to reopen the bridge has damaged its economy and cut off access to emergency services.

At the bridge, police eventually fired rubber bullets and “copious amounts of tear gas”, according to Nataanii Means, a 26-year-old member of the Oglala Lakota tribe, who filmed the confrontations.

“It was a lot of violence for unarmed water protectors that had a barricade between them and the police,” said Means, who posted footage showing one man who appeared to have a suffered a significant eye injury. After facing extensive tear gas, he added, “I fell on all fours. I was vomiting. I couldn’t see.”

Johnny Dangers, a photographer who frequently posts live videos from Standing Rock, captured footage of police near the pipeline drill pad.

“They’re ramping up,” he said in an interview, noting that he has not previously seen the national guard take such an “offensive” approach. “They just started pushing into people.”

Dangers’ video at one point showed a line of police holding large shields, with some pointing weapons and shooting what appear to be rubber bullets directly at activists feet away from them.

Lukban, who arrived two weeks ago, said the action were centered on prayer, but that she was quickly hit with pepper spray and a shield in the same part of her head.

“I don’t understand why anybody who is praying would be pepper sprayed or shot in the face,” she said, adding that children and elders were present.

Rob Keller, spokesman for the Morton County sheriff, said the individuals arrested face a range of charges, including trespassing, rioting, fleeing, assaulting an officer and preventing arrest. Other than pepper spray, he declined to identify the munitions used, but said in a statement: “Less than lethal weapons were used when the protesters were given multiple orders and warnings to move back.”

Keller said police have not received reports of serious injuries, but said an ambulance was called to assist two people arrested – one with a “minor cut” and one affected by pepper spray.

The sheriff’s office also published video of activists cutting a “security fence” and claimed some threw the wire at police.

William Prokopyk, spokesman for the national guard, said the shields are used to protect soldiers from “objects”, adding in an email, “These National Guard Soldiers received training on how to safely handle aggressive behavior in a controlled, not violent manner in an attempt to defuse the situation.”

LaDonna Brave Bull Allard, a Standing Rock tribe member who owns land where one of the main camps is located, said she knew of at least 10 people who were injured, adding, “There’s a lot of trauma.”

The violent clashes have erupted as lawmakers in the Republican-led state legislature have introduced several bills aimed at restricting anti-oil protests. One particularly controversial proposal would shield drivers from liability if they “unintentionally” injure or kill protesters blocking a public road.

Means said he feared police would be emboldened to escalate tactics after Trump steps into office, but added that indigenous people would continue to respond with peaceful actions.